Three windows buttons gone
This is weird, because opera, unlike chrome or chromium, does not have its own csd (client side decorations), so it relies on the window manager of the desktop enviroment for the minimise, maximise, close functions.
What window manager are you using?
Needless to say I have no problem here (debian testing x64 with openbox).
I have old Fedora 22 in VM, not updating. Only Opera is updating there. Nice couple of versions before, there were all buttons. After some time, only X was visible. Now, nothing is visible. I installed in second VM Fedora 26, complete default XFCE, with latest Opera, and now, only X is visible when maximized.
hmm have u changed flags? there's some flag regarding the title bar...
No, pure default installation. I didn't found any flag regarding titlebar or buttons.
Xubuntu 16.04 32 bit. Also have no buttons in the upper corner when Opera is maximized. I only have Tab menu here.
They do not want us to use their browser...
You can make it visible again, just follow the steps: go to Settings -> Window Manager Tweaks > Accessibility, and uncheck the checkbox on the left of "Hide frame of windows when maximized".
I had the issue of a maximized Opera "fighting" not do be un-maximized or resized using latest LinuxMint (Cinnamon) –
closewindow decorations were visible, though. Using a swift mouse move it was possible to move the maximized Opera from one Screen to an adjacent, different sized monitor where it instantly maximized itself again. All those mouse interactions leading to flickering, by the way.
I checked any of Cinnamon's "snap to *" settings but what did get me Opera restored was to save the session with V7 Session sidebar extension, closing Opera, deleting "
Current Session" from the profile, and ultimately restoring the session from V7 Sessions.
I guess, there is a bug – and perhaps it is the same the OP was suffering from.